Physiologic and familial-essential-senile (FES) tremors are poorly understood. Consequently, the treatment of FES tremor remains unsatisfactory. Although several authors have proposed a relationship between FES tremor and physiologic tremor, there is presently no conclusive evidence supporting such a relationship. This study, which is conducted in two experimental phases, will examine 30 FES tremor patients and 30 age-matched controls. During Phase I, tremor of the extended hand will be measured while maintaining a steady horizontal posture against gravity. During Phase II, the subject's hand, with fingers extended, will be inserted into a wedge-shaped manipulandum. The subject will then perform slow wrist extension and flexion movements while tracking a computer-directed target on an oscilloscope. The proposed experiment will examine the effects of mass loading on hand tremor and on the associated forearm EMG activity. Mass loading should lower the frequency of those tremors that are generated in the peripheral stretch reflex and should not alter the frequency of those tremors generated by central oscillators. Auto- and cross-spectral analyses will be performed on all tremor and EMG data in order to obtain accurate estimates of tremor amplitude, EMG amplitude, and timing relationships between hand oscillation and forearm extensor and flexor EMG activity. The relationships between stretch reflex gain and tremor amplitude will be measured by perturbing the hand/manipulandum with a computer-controlled torque motor, and the unloading reflex will be used to test the dependence of tremor rhythm on stretch reflex input. Because FES tremor may be a forme fruste of dystonia, this study will attempt to identify abnormalities in antagonistic muscle interaction during the postural tremor and during the slow tracking movements. FES tremor patients who exhibit synchronous EMG bursts in antagonistic muscles will be compared with those patients exhibiting alternating EMG bursts. Preliminary studies suggest that both patterns of antagonistic muscle interaction may exist in the same patient. Throughout the proposed study, particular attention will be directed toward the 8-12 Hz component of physiologic tremor which exhibits marked intersubject variability and which behaves like FES tremor during mass loading. Normal controls with and without prominent 8-12 Hz tremors will be compared with each other and with FES tremor patients who have tremor frequencies in the 8-12 Hz range. In particular, the study will investigate the possibility that the 8-12 Hz component of physiologic tremor is a forme fruste of FES tremor.